1. Field of the Invention
The invention relates to a method of mounting, to a printed circuit board, an electronic component having connection leads or pins that extend from the electronic component.
2. Description of the Background Art
FIGS. 4A and 4B illustrate a conventional method of mounting an electronic component, for example, a flat-package IC, as disclosed in Japanese Patent KoKoKu Publication No. 60-58600. FIG. 4A is a plan view showing solder paste as it has been applied to a printed circuit board. FIG. 4B is an enlarged perspective view of section A, which is encircled by a broken line in FIG. 4A.
In the figures, reference numeral 11 denotes a printed circuit board; 14a and 14b denote pads arranged on the printed circuit board 11 to form four rows 24 parallel to the four sides of an electronic component such as a flat-package IC device, which is to be mounted and which is not shown. Pads 14b which are positioned at the ends of the rows 24 are wider than pads 14a. The pads 14a and the wide pads 14b are arranged at an equal interval or pitch so as to allow contact with connection leads projecting at an equal interval from each side of the electronic component. Element 15 is a strip of solder paste applied linearly, uniformly, and in the direction of each pad row 24.
An electronic component such as a flat-package IC device is mounted on the printed circuit board 11 in the following manner. First, solder paste 15 is applied in a strip across each pad row 24. Here, the solder paste is applied linearly, uniformly, and, so that a suitable amount may be applied, with its width adjusted in accordance with the dimensions of pads 14a, the dimensions of wide pads 14b, and the dimensions of connection leads protruding from the IC device to be mounted. Next, the IC device connection leads are bonded to the corresponding pads (pad 14a or wide pad 14b) by reflow soldering.
In reflow soldering, the previously applied solder paste 15 agglomerates (draws in together) on each pad upon melting and through an action of surface tension; by this, solder near one pad draws away from solder near adjacent pads. Afterwards, the solder solidifies, thereby binding or fixing the IC device to the printed circuit board 11.
However, this conventional method of mounting an electronic component has the following problems the amount of solder paste to be the amount of solder paste to be applied is determined in accordance with the dimensions of the pads and the dimensions of the connection leads of the component and is adjusted by adjusting the width of the strip of solder paste applied in advance. Consequently, if the width of the strip is made too large (i.e., if the amount of solder paste applied is excessive), an excess of solder paste may occur in components; and upon reflow soldering, the solder may not agglomerate sufficiently on top of a pad, thereby causing "solder bridging" and necessitating corrective work after soldering. On the other hand, if, in order to prevent the occurrence of solder bridging, the width of the solder is made too small, there may occur an insufficiency of solder.
Furthermore, there is often a need to position, between adjacent ends of pad rows orthogonal to each other, other circuit patterns; however, should the last pad in each pad row be a wide pad, it is often difficult to obtain space sufficient to dispose such other circuit pattern.